G?s-Hedgehog signaling in intramembranous bone formation and expansion Summary The development of a functional skeletal system requires tight spatial and temporal control of osteoblast differentiation and maturation. How osteoblast cells are induced at the outset of bone development is a central question in understanding the organizational principles underpinning a functional skeletal system. Extraskeletal or heterotopic ossification (HO) occurs as a common complication of trauma or in rare genetic disorders and can be disabling and lethal. The precise cellular and molecular mechanisms underlying HO are not clear. Research in our lab has provided insights into the molecular and cellular regulation of bone development and recently we have identified a novel G?s-Hedgehog (Hh) signaling axis that critically regulates ectopic osteoblast differentiation in progressive osseous heteroplasia (POH). POH is a rare human genetic disease in which HO occurs predominantly through an intramembranous process and progresses from subcutaneous tissue into skeletal muscle and deep connective tissues. POH is caused by inactivating mutations in GNAS that encodes G?s that transduces signals from G protein coupled receptors (GPCRs). We have found that loss of G?s function in POH leads to ligand-independent activation of Hh signaling, which in turn induces osteoblast differentiation of mesenchyme cells in soft tissues, whereas activation of G?s signaling leads to Wnt/?-catenin signaling upregulation and reduced osteoblast differentiation in the human condition of fibrous dysplasia (FD). We have further observed in our preliminary studies that ectopic bone formation and expansion in POH bare cellular and molecular similarities to craniofacial bone development. Here we will build upon our unique perspectives and test our central hypothesis: Hh signaling activation by G?s inhibition induces osteoblast differentiation during intramembranous bone formation and recruits wild type cells into ectopic bone during progressive ossification in POH. In Specific Aim 1, we will investigate the role of G?s-regulated Hh signaling during formation and growth of intramembranous bone. In Specific Aim 2, we will extend findings in normal craniofacial bone growth to ectopic bone in POH. We will test our hypothesis that ectopic bone in POH expands by inducing a suture-like tissue where wild type osteogenic mesenchyme stem cells reside. Our proposed studies will provide an unprecedented level of insight in acquired HO and the regulation of osteoblast differentiation under both physiological and pathological conditions. Knowledge gained here from the mouse models of POH will be readily translatable to human diseases such as POH, acquired HO, FD, craniosynostosis and osteoporosis. We anticipate that our findings will have broad significance with respect to cell-fate specification and reprogramming processes during development, repair, and regeneration of many other organ systems where G?s-Hh and G?s-Wnt signaling plays a critical role and enhance our understanding of these signaling pathways in human diseases including cancer.